1. Field of the Invention
The present invention relates to an antenna mainly used for a mobile radio, and more particularly, to a mobile radio antenna preferably used for a base station.
2.Related art of the Invention
In recent years, mobile radios, such as cellular phones and personal handyphone systems (hereinafter called PHSs), have been widely used. Many small base stations, especially for PHSs must be built, since its base stations and mobile stations have low power. Thus such base stations are required to be reduced in space.
Preferably, antennas for mobile radio base stations are as horizontally omnidirectional as possible because their mobile stations cannot be located. It is also preferable that their beam tilt angle can be set between zero and a few degrees in a vertical plane except for special antennas including indoor antennas and that their gain be high. FIG. 24 shows an example of such a conventional antenna for mobile radio base stations, a two-element co-linear array antenna. As shown in FIG. 24, a radome 115, which is nonconductive and housing an antenna consists of a radome top 115a, a radome bottom 115b, and a radome wall 115c, with a coaxial feeder 112 installed between radome top 115a and radome bottom 115b. A first dipole antenna 109 is formed with an internal conductor 112a above the coaxial feeder 112 and with a metal pipe 113, held by a spacer 114 made of an insulating material, such as Fluoride resin, and powered through an external conductor of the coaxial feeder 112. A second dipole antenna 110 is formed by symmetrically positioning metal pipes 113, held by spacers 114, above and below a circular slit 112X, provided around the external conductor of the coaxial feeder 112. The second antenna is powered through the circular slit 112X.
In the above arrangement, the first and second dipole antennas 109 and 110 are vertical, and the diagram representing its directivity in a horizontal plane is virtually round. The dipole antennas also have high directivity in a vertical plane and provides a desired gain because they are stacked vertically. For the arrangement, the beam tilt angle depends on the distance between the feeding points of the first and second dipole antennas 109 and 110. To tilt radiation beams from the arrangement down (or toward the -Z direction), the distance is reduced. To tilt the radio beams up (or toward the +Z direction), the distance is increased.
Although such conventional antennas have characteristics necessary for antennas for mobile radio base stations, their vertical dimension will be inevitably large. A conventional antenna must have a vertical dimension of about 177 mm for 1.9 GHz, for example. A vertical diversity antenna is as long as about 572 mm, assuming that the distance between its upper and lower antennas is 2.5.lambda. (395 mm). Tilting beams from the antenna up causes it to become longer (its length increases from 177 to 191 mm when the tilt angle is set to +10.degree.), thus limiting the location of its installation.